1. Field of the Invention
This invention relates to improvements in inversion spin echo magnetic resonance imaging and, more specifically, it relates to a system which through control of certain perameters, will provide image contrast and feature detection which may be enhanced with respect to prior art practices while employing shorter scanning times.
2. Description of the Prior Art
The advantageous use of noninvasive and nondestructive test procedures has long been known in both medicine and industrial applications. In respect of medical uses, it has also been known that limiting a patient's exposure to potentially damaging x-ray radiation may advantageously be accomplished through the use of other noninvasive imaging procedures such as, for example, ultrasound imaging and magnetic resonance imaging. As to the latter, see, for example, The Fundamentals of Magnetic Resonance Imaging by Hinshaw et al. Technicare Corporation, 1984.
In a general sense, magnetic resonance imaging involves providing bursts of radio frequency energy on a specimen positioned in a main magnetic field in order to induce responsive emission of magnetic radiation from the hydrogen nuclei or other nuclei. The emitted signal may be detected in such a manner as to provide information as to the intensity of the response and the spatial origin of the nuclei emitting the responsive magnetic signal. In general, the imaging may be performed in a slice, or plane or multiple planes or three-dimensional volume with information corresponding to the responsively emitted magnetic radiation being received by a computer which stores the information in the form of numbers corresponding to the intensity of the signal. The Pixel value is established in the computer by employing Fourier Transformation which coverts the signal amplitude as a function of time to signal amplitude as a function of frequency. The signals are stored in the computer and may be delivered with or without enhancement to a video screen display such as a cathode-ray tube, for example, wherein the image created by the computer output will be presented through black and white presentations varying in intensity or color presentations varying in hue and intensity (and "saturation" or amount of "white" mixed in).
It has been known that magnetic resonance image intensity is dependent upon certain inherent physical properties of the tissues being investigated and timing intervals chosen by the user of the equipment. The physical properties of the tissues include the hydrogen density or density of the sensitive nucleus and two time factors which are known as T.sub.1 and T.sub.2. T.sub.1 which is also known as "T.sub.1 relaxation" is a measure of how long it takes the sample to regain its potential to produce a signal after a first pulse has caused it to respond to the pulsed RF excitation. This is sometimes considered as the time required to restore the longitudinal magnetization. T.sub.2 or "T.sub.2 relaxation" is a measure of the amount of time required for the magnetic resonance signal emitted by the radio frequency energy-excited proton to ideally dissipate to a point where it is generally imperceptible. At equilibrium, the transverse component of magnetization is at zero and the longitudinal component is equal to the initial magnetization. Decay to the former equilibrium is governed by the T.sub.2 relaxation and decay to the latter equilibrium is governed by the T.sub.1 relaxation. By properly selecting the timing intervals, the differences in hydrogen density or density of the sensitive nucleus (herein referred to as "N"), T.sub.1 and T.sub.2 values produce a difference in image intensity.
In general, in magnetic resonance imaging, it has been recognized that the differences in tissue T.sub.1 and T.sub.2 values are generally correlated such that an increase in one is accompanied by an increase in the other and a decrease in one is accompanied by a decrease in the other. Unfortunately, such parallel change does not contribute to a cooperative change in image intensity. For example, an increase in T.sub.2 causes an absolute increase in intensity while an increase in T.sub.1 usually decreases intensity. As a result, it is necessary to select the sequences so as to be dependent on either T.sub.1 or T.sub.2, but not both. The unfortunate consequence of such an approach is that many lesions or other elements desired to be visualized will go undetected if they only cause a significant change in one of the relaxation times T.sub.1 or T.sub.2. Also, competing T.sub.1 and T.sub.2 effects in respect of image intensity result in less contrast than would be the case if T.sub.1 and T.sub.2 effects were cooperative. Also, separate T.sub.1 sensitive and T.sub.2 sensitive pulse sequence acquisitions must be used to screen for both T.sub.1 and T.sub.2 changes, resulting in lengthy clinical studies.
It has been known to use spin echo and inversion spin echo techniques in magnetic resonance imaging. See generally pages 32 through 50 of the hereinbefore cited Hinshaw et al. publication.
In conventional spin echo imaging procedures, after an initial 90 degree pulse or general alpha-degree pulse, there are at predetermined intervals 180 degree RF pulses or magnetic field pulses which serve to refocus the transverse magnetization after the signal from the nuclei disappears to thereby cause the signal to reappear. These pulses which effect a spin echo will herein be referred to as "refocussing pulses". This regenerated signal is referred to a "spin echo". Depending upon whether the 180 degree pulses are in phase or 90 degrees out of phase with the 90 degree pulse, the resultant signals will either be solely positive or alternate between positive and negative. To the extent to which T.sub.2 relaxation has occurred prior to the generated spin echo, that portion of the signal is irretrievably lost.
In inversion recovery spin echo practices, there is an initial 180 degree pulse followed after a predetermined time period by a 90 degree pulse followed by a further time period and a repeat of the 180 degree spin echo cycle. This results in a potential for negative signals and a spreading out of the longitudinal magnetization thereby permitting images with greater dynamic range of contrast. The time between the first 180 degree pulse which inverts all magnetization and the subsequent 90 degree pulse which initiates the reading of the recovered longitudinal magnetization is deemed to be the "inversion time" which will be referred to herein as "TI".
It has been known in inversion-spin echo sequences of short TI intervals to provide additive T.sub.1 and T.sub.2 effects. See R. E. Steiner et al., Society of Magnetic Resonance in Medicine (1985), page 1208. The difficulty with this sequence is that it requires TI intervals which are much shorter than T.sub.1 so that the signals remain negative. As a result, T.sub.1 sensitivity is sub-optimal as it is maximal when TI equals T.sub.1 and contrast may be reduced even though T.sub.1 and T.sub.2 effects are additive. Moreover, the longitudinal magnetization recovers to nearly a zero magnitude during the short TI interval resulting in low signal to noise ratios.
It has been know to evaluate with mathematical models the desired pulse sequences and timing intervals in magnetic resonance imaging employing spin echo and inversion spin echo pulse sequences. See Mitchell et al. INVESTIGATIVE RADIOLOGY, Vol. 19, No. 5, pp. 350-360 (1984).
U.S. Pat. No. 4,254,778 discloses a system for driven equilibrium wherein after data collection, long TD intervals are sought to be avoided by applying a 90-180-90 triplet with the second 90 degree pulse shifted 180 degrees and the 180 pulses shifted 90 degrees in phase with respect to the first 90 degree pulse. This is said to drive the transverse magnetization at the spin echo peak back up to the positive longitudinal direction to thereby hasten recovery of longitudinal magnetization. This prior system has as its primary objective shortening of overall repetition time i.e., to produce maximal intensity in the shortest time. This is undesirable for certain types of imaging such as medical imaging as intensity is maximized at the expense of image contrast e.g., there is no sensitivity to differences in T.sub.1 as longitudinal magnetization is restored instantly regardless of T.sub.1 value. See also, Jensen et al., Medical Physics 14, 38-42 (1987) which discloses a 90-180-90 pulse triplet for saturating the signals outside of a plane. This triplet employs RF signals to drive the spin echo signal to the positive longitudinal direction rather than the negative direction of the present invention.
In spite of the foregoing, there remains a very real and substantial need to provide inversion spin echo procedures which will facilitate improved contrast with relatively short scanning periods.